1. Field of the Invention
The present invention relates to an improved servo control circuit for controlling the operation of an injection molding apparatus.
2. Description of the Prior Art
The present invention is an improvement of the servo control circuit described in U.S. Pat. No. 5,316,707 to Stanciu et al., which issued on May 31, 1994 (hereinafter xe2x80x9cthe ""707 patentxe2x80x9d) which is incorporated herein by reference.
The ""707 patent discloses an injection molding apparatus which is controlled by a control system for controllably injecting fluid molding material into a die. The control system disclosed in the ""707 patent includes, among other things, a programmable logic controller (PLC) and a servo control circuit. The servo control circuit receives control signals from the PLC and receives feedback signals from the injection molding apparatus related to the pressure of the fluid molding material in the die and the velocity the fluid molding material is injected into the die. The servo control circuit combines the input signals and the feedback signals in a manner that enables the fluid molding material to be injected into the die in a reliable, repeatable manner.
A problem with this servo control circuit is that during a first part of an injection cycle a pressure feedback signal related to the pressure of the fluid molding material in the die interferes with the attainment of the maximum injection velocity of the fluid molding material into the die even though the actual pressure of the fluid molding material in the die is at or near atmospheric pressure. Moreover, during a second part of the injection cycle, a velocity feedback signal related to the velocity the fluid molding material is injected into the die interferes with the maintenance of a constant pressure of the fluid molding material in the die even though the fluid molding material in the die is not moving. Still another problem with the control system disclosed in the ""707 patent is the inability to switch the control mode implemented by the servo control circuit to optimize the velocity of the injected fluid molding material during the first part of the injection cycle and/or the maintenance of the pressure of the fluid molding material in the die during the second part of the injection cycle.
One attempt at overcoming these problems included programming the PLC to selectively control a transfer point between the first part of the injection cycle where only the injection velocity of the fluid molding material into the die is controlled and the second part of the injection cycle where only the pressure of the fluid molding material in the die is controlled. A problem with this solution, however, is that a typical PLC does not have a sufficiently short loop control time in order to enable it to control the injection molding cycle with the precision needed. Moreover, in this embodiment, the pressure feedback signal still adversely affected the velocity of the injected fluid molding material during the first part of the injection cycle and the velocity feedback signal still adversely affected the pressure of the fluid molding material during the second part of the injection cycle.
It is, therefore, an object of the present invention to overcome the above problems and others by providing an improved servo control circuit that is responsive to control signals from the PLC for controlling the injection cycle whereby the control circuit implements a first control mode which controls only a velocity of the fluid molding material during a first part of the injection cycle and a second control mode which controls only a pressure of the fluid molding material during a second part of the injection cycle. Still other objects will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.
Accordingly, I have invented an injection molding apparatus including means for injecting fluid molding material into a die and a control system for controlling the means for injecting. The control system includes a programmable logic controller (PLC) for generating control signals and a servo control circuit responsive to the control signals for implementing an integral only control mode or a proportional only control mode for controlling the injecting means during an injection cycle wherein, during a first part of the injection cycle, the servo control circuit operates in the integral only control mode and during a second part of an injection cycle the servo control circuit operates in the proportional only control mode.
I have also invented an injection molding apparatus including a moveable injection ram for urging fluid molding material into a die. The movement of the injection ram is controlled by a software controlled controller and a servo control circuit which senses movement of the injection ram and a pressure of the fluid molding material in response to the injection ram urging the fluid molding material into the die during an injection cycle. The servo control circuit is responsive to the controller for controlling only a velocity of the injection ram during a first part of the injection cycle and for controlling only a pressure the injection ram exerts on the fluid molding material during a second part of the injection cycle.
Lastly, I have invented an injection molding apparatus that includes means for injecting fluid molding material into a die and a control system for controlling the means for injecting. The control system includes a controller for generating control signals and a servo control circuit responsive to the control signals for controlling the injecting means. The servo control circuit implements an integral control mode which controls only a velocity of the injecting means during a first part of the injection cycle and a proportional control mode which controls only a pressure exerted by the injecting means on the fluid molding material during a second part of the injection cycle.
During the first part of the injection cycle, when the die is initially being filled with fluid molding material, the servo control circuit is responsive to the velocity the fluid molding material is being injected into the die and a reference pressure signal, but not an actual pressure of the fluid molding material, for causing the injecting means to inject the fluid molding material into the die at a velocity related to the actual velocity signal. During the second part of the injection cycle, when the die is substantially filled with fluid molding material, the servo control circuit is responsive to the actual pressure of the fluid molding material in the die and a reference velocity signal, but not the actual velocity of the fluid molding material, for causing the injecting means to maintain the pressure of the fluid molding material at a pressure related to the actual pressure of the fluid molding material in the die.
The servo control circuit can include a switched integrator responsive to the actual velocity of the fluid molding material during the first part of the injection cycle and the reference velocity signal during the second part of the injection cycle for controlling the injecting means during the injection cycle. The switched integrator switches between an integrating mode during the first part of the injection cycle and a buffered reference mode during the second part of the injection cycle.
The servo control circuit preferably includes a velocity feedback isolation circuit responsive to the actual pressure of the fluid molding material and an external reference pressure signal for causing the servo control circuit to change from the first part of the injection cycle to the second part of the injection cycle when the actual pressure signal during the injection cycle achieves a predetermined relation to the external pressure reference signal.
Lastly, the servo control circuit preferably includes a pressure error circuit responsive to the actual pressure signal and the response of the velocity feedback isolation circuit to the actual pressure signal and the external reference pressure signal for outputting a pressure control signal related to the actual pressure signal or the external reference pressure signal.